1. Technical Field
The present invention relates to bearing materials used for rolling bearings, such as roller bearings and ball bearings. In particular, the present invention relates to a bearing material having an extended rolling contact fatigue life.
2. Background Art
Bearings require excellent durability against fatigue, and thus bearing materials also require an extended rolling contact fatigue life.
One of generally known methods for extending the rolling contact fatigue lives of bearing materials is reduction of nonmetal oxide inclusions contained in the materials. Such reduction of the nonmetal oxide inclusions have been achieved by decreasing the oxygen content in the materials. At present, the oxygen content in the materials can be decreased to 10 ppm or less by weight due to the progress in refining technologies. The reduction of the oxygen content, however, has reached its limit, and further extension of the rolling contact fatigue life is not expected by such a method.
Under such circumstances, several methods other than reduction of the oxygen content have been proposed for extending the rolling contact fatigue life. For example, Laid-Open Patent No. 3-126839 discloses a bearing material having an extended life which is achieved by the reduction of the number of nonmetal oxide inclusions per unit area or volume. Japanese Laid-Open Patent No. 5-25587 discloses a bearing material having an extended life which is achieved by the reduction of a maximum diameter of nonmetal oxide inclusions, which is estimated from statistics of extreme-value. Further, Japanese Laid-Open Patent No. 4-280941 discloses a bearing steel having an extended rolling contact fatigue life, in which the length and number of nonmetal sulfide inclusions in the steel are controlled so that the inclusions having a maximum length of not greater than 100 .mu.m and the number of the inclusions each having a length of greater than 40 .mu.m are 50 or less in an observation area of 160 mm.sup.2.